Metal particles and method of making



March 29, 1960 Filed Aug. 26, 1957 V L. J. NOVAK METAL PARTICLES ANDMETHOD OF MAKING HEAT SOL/O PARTICLES IN INERT' AT OSPHERE CONTACTPARTICLES WITH CASEOUS METAL COMPOUND PARTICLES WITH SOLVENT LEACH METALCOATED J I SEPARATE LEACHEO PART/CLESI RECOVER HOLLOW IRREGULAR SHAPEDMETAL PARTICLES I 2 Sheets-Sheet 1 LRECOVER METAL PART/CLES| my THEPARTICLES l INVENTOR LEO J. NOVAK BY 7.... 4 law ATTORNEYS March 29,1960 L. J. NOVAK METAL PARTICLES AND METHOD OF MAKING 2 Sheets-Sheet 2Filed Aug. 26, 1957 N WH INVENTOR LEO J. IVOVAK ATTORNEYS United StatesPatent METAL PARTICLES AND METHOD OF MAKHIG Leo J. Novak, Dayton, Ohio,assignor to The Commonwealth Engineering Company of Ohio, Dayton, OhioApplication August 26, 1957, Serial No. 680,311

8 Claims. (Cl. 252-477) This invention relates to the art of gas platingand wherein metal coatings are deposited by thermal decomposition of agaseous metal compound and which are heat-decomposable in contact withthe article or substrate to be plated with the metal constituent of thecompound. Such gas plating processes have been found to have particularutility for depositing metal coatings at temperatures far below themelting points of the metals. The processes are also operable at normalor sub-normal atmospheric pressures.

It is an object of'the present invention to gas plate particles whicharev solid and which may be utilized as a catalyst in the carrying outof various chemical processes.

It is another object of the invention to provide an article by gasplating particles such as in the form of crystals and which crystals aregas plated to provide a thin, porous film of metal over the crystals andthereafter said gas plated crystals are leeched with a solvent of thecrystals so as to dissolve the same out and leave a hollow, relativelyporous shell of the metal deposited by gas plating. Such products may beused as a catalyst or as fine particles of metal. Such fine, dividedparticles of metal may also be used in the making of sintered metalproducts such as bearings and the like.

Another object of the invention is to produce a product whereinparticles of solids such as composed of inorganic materials such assilica, sand, carbon and the like, and which are coated with a shell ofmetal by gas plating to provide a product which is useful where powderedmetal or the like is desired.

These and other objects and advantages of the invention will becomeapparent from the following description taken in conjunction with thedrawings, wherein Figure 1 is a flow sheet illustrating the steps ofappli-, cants process for gas plating solid particles to produce hollow,irregular shaped metal particles;

Figure 2 is an enlarged cross section of a crystal of sodium chloridegas plated to provide the same with a thin shell or coating of nickel;

Figure 3 is a similar view in cross section of a metal particle whichhas been leached after gas plating a crystal of sodium chloride todissolve out the sodium chloride and leaving a metal shell;

Figure 4 illustrates an apparatus embodiment for carrying out theprocess, including the steps of gas platting the particles and leachingthe same, the apparatus being illustrated in cross section to betterillustrate its construction;

Figure 5 is a cross section taken substantially on the line 55 of Figure4 and looking in the direction of the arrows; and

Figure 6 is an enlarged cross section of a perforated metal shellparticle which has been made by gas plating to provide a porous metalshell.

In carrying out the invention, solid particles which are to be gasplated are introduced into a receptacle or compartment and heated to atemperature suflicient to ther- "ice mally decompose the gaseous metalcompound brought in contact therewith, and which compound comprises themetal which is to be deposited on the particles. The particles, afterbeing gas plated, are recovered and provided the same are to-be leached,are deposited in a leaching bath containing solvent for the solid orcore portion of the articles. I

The gas plated particles are treated with a solvent such as water as inthe case of crystals which are soluble in the water so as to dissolveout the core portion and leave leached, irregular shaped particles ofmetal. These are then recovered from the leaching solution and allowedto dry, or are force air dried, to provide the porous, irregular shapedparticles of metal.

The method steps are illustrated in Figure 1, and wherein the dryparticles are recovered whichare either solid throughout, having a coreformed of the substrate, or where the particles have been gas plated andleached, then hollow, irregular shaped metal particles are recovered, asshown in the flow sheet of Figure 1.

Metals which may be deposited as thin films, are for example, nickel,chromium, copper, tungsten, tin, zinc,

lead, and the like.

One particularly advantageous method of bringing metal as vapors intothe plating zone is in the form of readily decomposable gaseouscompounds. For example, the metals may be introduced as gaseous metalcarbonyls, also nitroxyl compounds, nitrosyl carbonyls, metal hydrides,metal alkyls, metal halides, and the like.

Illustrative compounds of the carbonyl type are nickel, iron, chromium,molybdenum, cobalt, and mixed carbonyls.

Illustrative compounds of other groups are the nitroxyls, such as coppernitroxyl; nitrosyl carbonyls, for example, cobalt nitrosyl carbonyl;hydrides, such as antimony hydride, tin hydride; metal alkyls, such astetraethyl lead; metal halides, such as chromyl chloride; and carbonylhalogens, for example, osmium carbonyl bromide, ruthenium carbonylchloride, and the like.

In the preparation of ionic crystals, to which the invention is adapted,crystals such as formed of sodium chloride, magnesium sulfate, sodiumnitrate and the like are subjected to gas plating, as for example nickelcarbonyl or the carbonyls of platinum, rhodium or the like, and metalacetylacetonates. The crystals are placed in a chamber or container andwhile heated to a temperature up to 600 C. or such as to be below thedecomposition temperature of the crystals and yet be above the thermaldecomposition temperature of the gaseous metal compounds and thecrystals are subjected to gas plating with these thermally decomposablegaseous metal compounds.

After gas plating the crystals with the metal the same are then removedand subjected to leaching with water to remove the core and recover theporous metal shell portion of the particles.- In this manner catalystsmade of platinum, rhodium, copper, nickel and the like are readilyobtained and wherein the particles are hollow,

irregular shaped and provide a relatively large surface for contactduring catalytic processes.

Other crystals which are adapted to be gas plated in accordance withthis invention may comprise organics, plastics and the like, such as aresoluble in organo s01- vents such as ether, dioxane, benzene, etc. As anexample, petroleum wax particles such as formed of hard waxes, forexample, Carnuba wax, are gas plated and then leached with benzene,kerosene or the like aliphatic or aromatic solvent so as to dissolve outthe wax and to leave the metal. In this instance the gas plating iscarried out at a temperature of around 250 F. and such as to preventundue softening of the wax particles. Other organic substrate formingparticles may comprise solid Example 1 Salt crystals are plated withnickel carbonyl under the following conditions and utilizing argon asthe inert carrier gas, together with a small proportion of ammonia (NH'The gas plating was conducted for a time of 20 minutes as follows:

Plating cycle- 4000 cc./m. argon 15 cc./m. NH 300 cc./m. Ni(C). 340 F.temperature Time 20 minutes Example 2 The above process is carried outas in Example 1, using sand and wherein leaching process is eliminatedto recover a sand particle having an outer shell of nickel metal. Theprocess of the gas plating in this instance using the sand particles,was as follows:

Platingcycle- V 3000 cc./m. argon cc./m. NH 300 cc./m. Ni(CO) 370- 380F. temperature 30 minutes plating time Example 3 In this examplemagnesium sulfate crystals were plated with copper, utilizing the samecarrier gas and process with nickel carbonyl, as set out in Example 1.In this instance the metal coating is copper, using copperacetylacetonate as the gaseous metal thermally decomposable metalcompound.

Example 4 In this instance particles ofcrystal salt are plated withaluminum by the use of aluminum triisobutyl dissolvedv in heptane. Theapparatus is first freedof air by flowing nitrogen (dry) through theapparatus or platingchamber for aboutminutes to produce a platingchamber free of air.

Aluminum triisobutyl is carried into the plating chamber from a bubblertube containing the compound dissolved in heptane. .The aluminumtriisobutyl is carried into the plating chamber bya nitrogen carriergas. The

plating chamber is heated to a temperature of around approximately 563F. to cause the aluminum triisobutyl to heat decompose and deposit thealuminum. Thereafter the particles of salt comprising a shell ofaluminum are leached to remove the salt and recover a relatively porousshell of aluminum.

Example '5 In this instance platinum carbonyl is used to form a porousplatinum irregular shaped particle and wherein salt sodium'chloridecrystals are used to receive the plating of platinum. The gas plating iscarried out similarly as in Example 1, and the particles are leached torecover porous platinum irregular shaped particles.

Example 6 In this instance carbon particles are coated with molybdenumusing molybdenum carbonyl in place of nickel carbonyl and carrying outthe process similarly as in Example 2.

A suitable apparatus for carrying out the gas plating of particles madeof crystals, and wherein the same are to be leached is disclosed inFigures 4 and 5. Referring to the figures, a rotary kiln 10 is utilizedas a gas plating chamber, the same being adapted to be rotated in theconventional manner and using a motor 11 and driven pinion gear 12 whichin turn meshes with ring gear 13. The ring gear 13 is suitably securedto the upper end of the kiln 10 and is adapted to be mounted on thebearing 14. The kiln proper is supported on the columns 15 and 16 whichare provided with roller bearings 17 and'18 respectively.

'To introduce-the particles or crystals into the kiln, the same'areplaced in the hopper 20 which is provided with a gate 21 in thebottomportion thereof,.which is swingable to introduce particles or crystalsto be gas plated into the chamber 22 of the kiln. The hopper 20 isconnected through a lower adapter portion 23 suitably supported as bythe member 25, and is arranged to communicate to the interior of thekiln through the lower cylindrical portion 26, as shown in Figure 4.Suitable bafile members.28 are provided in the kiln which, as shown inFigure 5, are shifted to agitate the particles as the kiln 10 isrotated.

To introduce the gaseous metal thermally decom-' posable compound intothe chamber 22 of the kiln, the

same is brought in through the conduit 30, the same being mounted in theadapter portion providedto close the"v are conducted out of the systemthrough the conduit 37 which is suitably connected to the adapter 23 atthe head of the kiln. This conduit 37 may be a flexible connection, asindicated in the drawing, and where desired conduit 37 may be connectedto a vacuum pump or blower to provide suction where-the same is desired.

The bottom gate or closure member 21 is suitably provided with sealinggasket means 40 to prevent gaseous products from passing out into theupper hopper portion 20. Gaskets for-med of sponge rubber or flexibleplastic hose elements are suitable to seal off the kiln.

In order to heat the kiln, suitable heaters such as shown at 41 and 42are provided and which preferably are resistance heaters and heated byan electric source, not shown. Other heating means, of course, may beused, such as a gas burneror the like, and whereby the interior ofthekilnrchamber 22 is heated sufficiently so that the particles, asshown at 45, passing through the kiln are heated at a temperature highenough to cause heat decomposition of the gaseous metal compoundintroduced into the kiln chamber through the conduit 30, asaforementioned. I i l r The leaching solution 34 will depend upon thecore or substrate part of the particles being gas plated. Where theparticles are soluble in water, then the leaching solution will bewater, andwhen the particles are not soluble in water and are soluble inorganic compounds such as petroleum solvents as aforementioned, then theleaching solution will be petroleum solvent, e.g., benzene or the like.Also aliphatic organic solvents may be used, for example ethyl alcohol,acetone and the like. Other organic solvent compounds, preferably of theshort chain .(2 to 1'0 carbons) type, and which are commerciallyavailable may be used to dissolve the core and leave ing will accumulatein the bottom of the leaching tank, as at 48. The same will be recoveredby draining ofi the leaching solution and centrifuging the particles toremove retained leaching solution and the particles are then air driedas by the use of a blower which delivers warm air (80100 F.) onto theparticles.

The particles, as shown in Figure 2, provide a substrate or core whichis coated with a porous shell of metal, such as shown at 50 in Figure 2,and 51 in Figure 3. -In Figure 2 the solid particle is unleached whereasin Figure 3 the particle illustrated is such as after leaching away ofthe core so as to leave an open space as at 52.

In the gas plating process, control of the metal deposit is accomplishedby (a) regulating the carrier or inert gas concentration, (b) rate offlow of the plating gas atmosphere, and (c) temperature of the core orobjects being gas plated. The inert carrier gas useful for carrying outthe gas plating includes as well as argon, helium, carbon dioxide,hydrogen, nitrogen and the like. A thickness of metal deposit of about0.001 inch is achieved in approximately three to fifteen minutes,depending upon the gaseous metal compound used and its concentration asdescribed.

The gas plating deposits metal into the pores and interstices of thesubstrate material, and the adherence of the plated metal to thesurface, when free of foreign matter, is tenaceous and of uniformthickness.

In the preparation of gas plate shell particles for use as catalyst, thesolid portion forming the core, which is gas plated and then leached toprovide a metal shell, may be in the form of powder, e.g., particleshaving a size of less than one micron, e.g., 0.00015 to 0.01 micron.Further, where the catalyst is desired in oxide form, the nascent gasplated metal deposit may be heated in the presence of air or oxygen toform the oxide.

'The formation of the metal particles in accordance with this inventionby gas plating a leachable core or substrate, produces a metal particlehaving a porous outer surface as well as an inner surface which isrelatively porous and irregular. A metal particle is thus producedhaving a very large surface area, and many times that produced by asolid spherical or irregular shaped solid particle. This is due to theouter and inner porous wall structure of the leached gas plated shell ofeach metal particle.

The gas plating process of the invention provides for the production ofparticles having an outer shell or coating of metal, the particles beingof a low micron and preferably submicron size. The smaller the particlesize, the greater the contact surface area. Considering for examplespherically shaped particles, the enormous increase in total area as theparticle size decreases is shown by the following table:

No. of Spheres Total Area,

Sphere Diameter Centimeters 2 sures from 0.01 to 0.1 millimeter ofmercury may be maintained in the plating chamber during gas plating.

Each material from which a metal may be plated has a temperature atwhich decomposition is complete. However, decomposition may take placeslowly at a lower temperature or while the vapors are being raised intemperature through some particular range. For example, nickel carbonylcompletely decomposes at a temperature in the range of 375 F. to 400 F.However, nickel carbonyl starts to decompose slowly at about F. and,therefore, decomposition continues during the time of heating from 200F. to 380 F.

A large number of the metal carbonyls and hydrides may be effectivelyand efliciently decomposed at a temperature in the range of 350 F. to450 F. When working with most metal carbonyls we prefer to operate in atemperature range of 375 F. to 425 F.

Maintenance of the object at temperatures generally in the decompositionrange is readily accomplished by causing the object to be heated byinfra-red rays or by induction heating. The advantage of this type ofheating is its ready control within the temperature ranges utilized inthe process. These temperatures generally range from 350 F. to 450 F. inthe plating zones and from 800 F. to 1200 F. in the annealing zones.

To produce a porous metal shell, as illustrated in Figure 6, the gasplating is controlled so that a very thin film metal deposit forms onthe leachable solid substrate material. For example, by exposing thesubstrate to ten to twenty seconds with the thermally decomposable metalcompound, a layer of 0.0001 to 0.00025 inch is formed which is porous.By terminating the gas plating after depositing this thin porous metalcoating when the solid core or substrate is leached, a porous metalshell is obtained. Such a porous metal shell particle has increasedsurface area which is important when the metal particles are to be usedas a catalyst.

It will be understood that while the method and apparatus disclosed anddescribed herein illustrates a preferred form of the invention,modifications may be made therein without departing from the spirit andscope of this invention, and such modifications as occur to thoseskilled in the art are intended to be covered in the appended claims.

What is claimed is:

l. A method of producing metal particles which comprises establishing asource of dry inert gas and a source of thermally decomposable gaseousmetal compound, heating solid particles to be gas plated in an inertatmosphere, contacting the particles with the gaseous metal compoundwhile the particles are heated to a temperature high enough to thermallydecompose the gaseous metal compound and depositing the metal onto theheated particles, leaching the resultant metal coated particles with asolvent which dissolves the inner core of the particle leaving the outermetal shell, and recovering the metal shell particles.

2. A method of producing porous metal particles in finely dividedcondition, comprising the steps of providing solid particles of thematerial which is leachable, heating said particles in an inertatmosphere, contacting the heated particle with gaseous metal compoundwhile the temperature of said particles is high enough to cause saidgaseous metal compound to decompose and deposit the metal onto thesurface of the particles, thereafter leaching the gas plated particleswith solvent which dissolves the solid particles leaving metal shellparticles.

-31A method of producing porous metal particles in" leaching the gasplated particles with solvent which dissolves the solid particlesleaving metal shell particles, mechanically separating said leachedmetal particles from the leaching solution, and drying the particles toremove residual leaching solvent.

4. A method of producing porous metal particles of irregular shape,comprising subjecting a leachable solid to gas plating by thermallydecomposing the gaseous metal compound brought in contact therewith,leaching the resultant metal plated particles with a solvent to dissolvethe solid core leaving a shell of metal, and separating the metal shellfrom the leaching solvent.

5. A method of producing metal particles of irregular shape, comprisingsubjecting a leachable solid to gas plating by thermally decomposing agaseous metal carbonyl brought in contact therewith, leaching theresultant metal plated particles with asolvent to dissolve the solidcore leaving a shell of metal, and separating the metal shell from theleaching solvent.

6. A method of producing metal particles of irregular shape, comprisingsubjecting a leachable solid to gas plating by thermally decomposingorgano metal compounds brought in contact therewith, leaching'the re- 1sultant metal plated particles with'a solvent to dissolve the solid coreleaving a shell of metal, and separating the metal shell from theleaching solvent. a

7. A process for producing a metal catalyst comprising the steps ofheating solid particles which are leachable by a solvent selected frornthe group consisting of water, aliphatic organic solvent, and aromaticorganic solvent, subjecting the said particles to gaseous metal platingby heating the particles and contacting the same with a thermallydecomposable gaseous metal compound to plate the same with metal,leaching the resultant gas plated particles with said solvent to providemetal particles,'and separating said metal particles from the leachingsolvent to recover a powdered metal catalyst."

8. An article of manufacture produced in accordance with the process ofclaim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,382,371 Utterbach Aug. 14, 1945 2,599,978 Davis June 10, 19522,702,524 Fruth Feb. 22, 1955 2,798,051 Bicek July 2, 1957 2,817,311Nack Dec. 24, 1957 OTHER REFERENCES S61. Ne. 357,989, 'Brendlein(A.P.C,), published May 25, 1943.

1. A METHOD OF PRODUCING METAL PARTICLES WHICH COMPRISES ESTABLISHING ASOURCE OF DRY INERT GAS AND A SOURCE OF THERMALLY DECOMPOSABLE GASEOUSMETAL COMPOUND, HEATING SOLID PARTICLES TO BE GAS PLATED IN AN INERTATMOSPHERE, CONTACTING THE PARTICLES WITH THE GASEOUS METAL COMPOUNDWHILE THE PARTICLES ARE HEATED TO A TEMPERATUURE HIGH ENOUGH TOTHERMALLY DECOMPOSE THE GASEOUS METAL COMPOUND AND DEPOSITING THE METALONTO THE HEATED PARTICLES, LEACHING THE RESULTANT METAL COATED PARTICLESWITH A SOLVENT WHICH DISSOLVES THE INNER CORE OF THE PARTICLE LEAVINGTHE OUTER METAL SHELL, AND RECOVERING THE METAL SHELL PARTICLES.